1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) device and a method for manufacturing the organic EL device and more particularly to the organic EL device in which device main components having at least one organic light emitting layer and a cathode and containing oxygen in an interface between the organic light emitting layer and the cathode.
The present application claims priority of Japanese Patent Application No. 2000-317165 filed on Oct. 17, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
As one type of an EL device used in display units for information devices or a like, an organic EL device is being developed. FIG. 10 is a diagram schematically showing configurations of a well-known and general organic EL device.
As shown in FIG. 10, the general organic EL device includes a transparent insulating substrate 51 made of a glass substrate or a like, device main components having an anode 52 (lower electrode) made of a transparent conductive material such as ITO (Indium Tin Oxide) formed on the transparent insulating substrate 51, a hole transporting layer 53 formed on the anode 52, an organic light emitting layer 54 formed on the hole transporting layer 53, a cathode 55 (upper electrode) made of AlLi (aluminum lithium) or a like formed on the organic light emitting layer 54, and a cap 57 made of glass or a like mounted, with encapsulating resin 56 interposed between the transparent insulating substrate 51 and the cap 57, on the transparent insulating substrate 51 so as to cover the main components formed on the transparent insulating substrate 51. As the encapsulating resin 56, for example, a UV (Ultra-violet rays) curable resin is used. By applying light containing UV fed from a light source to the encapsulating resin 56, the encapsulating resin 56 is cured so as to carry out encapsulation.
Since a state at an interface between the organic light emitting layer 54 and the cathode 55 is not perfect, an unstable defect exists. That is, the defect here represents an impurity level caused by a lattice defect or a like existing at a place where an interface, level should be formed. Due to this defect, in addition to a path through which a carrier has to flow originally, another path is produced, which causes occurrence of a leakage current. Furthermore, there is a danger that the cathode 55 is shorted to the anode 52. As a result, properties of the organic EL device become unstable, thus making impossible to obtain a high rectification ratio and, therefore, when the organic EL device is driven in a simple matrix manner, a pixel short and/or crosstalk occur.
Here, a perfect state of interface means a state in which there is no level derived from the defect in the interface level at the interface between the organic light emitting layer 54 and the cathode 55 and there is a state in which an electron implantation can be smoothly performed by a heat exciting current, or a state in which a level that can induce a tunnel effect exists in a stable state. In contrast, an imperfect state of the interface means a state in which many interface levels are formed or vanished repeatedly due to the occurrence of the defects, causing variations in implanting characteristics. Therefore, it is necessary for the organic EL device to have the perfect state of the interface between the organic light emitting layer 54 and the cathode 55 and stable interface level. This enables an increase in the leakage current to be inhibited and the short between the cathode 55 and anode 52 to be avoided, thus the properties of the organic EL device to be made stable.
To solve this problem, an organic EL device is disclosed in, for example, Japanese Laid-open Patent Application No. Hei 11-312580 in which device elements such as the organic light emitting layers or a like are encapsulated in an atmosphere of oxidative gas to make stable the characteristic of the organic EL device. The disclosed organic EL device includes, as shown in FIG. 11, a glass substrate 61, an anode 62 made of ITO formed on the glass substrate 61, an organic film 63 having, for example, a stacked layer containing a hole transporting material and a light emitting layer formed on the anode 62, a cathode 64 having a metal consisting of MgAg (magnesium silver) or stacked layers of LiF (lithium fluoride) and Al (aluminum) on the organic film 63, and an enclosure 66 encapsulating the device elements (that is, the anode 62, organic film 63, and cathode 64) in a manner so as not to stick to the device element using an encapsulating resin 65 made of a UV curable resin or a like.
Gas containing oxidative gas is injected into the encapsulated space. The gas includes mixed gas of oxidative gas (O2, N2O or a like) and non-oxidative gas (inert gas such as N2, Ar, He, or a like). A concentration of the oxidized gas is set to 0.1 to 20%. Here, injection of the mixed gas is performed in a manner that the glass substrate 61 on which the device elements are formed is carried from a deposition chamber into a pre-degassing chamber in a vacuum state in which the enclosure 66 has been set and the mixed gas is introduced into the pre-degassing chamber until a pressure of the mixed gas becomes at an atmospheric level and the encapsulating resin 65 is cured by a UV lamp to complete the encapsulation.
By configuring as above, even if a short occurs between the anode 62 and the cathode 64 due to particles existing therein, since the particles are oxidized by the oxidative gas and go insulative, an insulating state between the anode 62 and the cathode 64 is restored and a life of the organic EL device is improved.
However, the method for manufacturing the organic EL device disclosed in Japanese Laid-open Patent Application Hei 11-312580 presents a problem. That is, in the disclosed method, since considerations are given only to a method of preventing the short between the anode 62 and cathode 64 due to particles, it is difficult to obtain a high rectification ratio without reducing a throughput for manufacturing the organic EL device.
That is, in the disclosed method, the life of the organic EL device is improved by introducing the mixed gas of the oxidative gas and non-oxidative gas before being encapsulated and by oxidizing the particles existing between the anode 62 and cathode 64 using the oxidative gas. In this case, as described by the paragraph number 0025 in Japanese Laid-open Patent Application Hei 11-312580, the cathode 64 of the organic EL device is formed so that its thickness becomes as comparatively large as 180 nm and the cathode 64 having the large thickness makes it impossible to provide the high rectification ratio. Therefore, if the rectification ratio has to be made larger, since the cathode 64 has to be formed so as to have a smaller thickness, a yield in the manufacturing of the organic EL device becomes lower, thus inevitably causing decrease of the throughput.